(Field of Technology)
The present invention generally relates to a plastics injection molding technique and, more particularly, to an injection molding apparatus for the manufacture of fiber-reinforced plastics moldings.
Japanese Patent Publication No. 55-133936, published Oct. 18, 1980, discloses a reaction injection molding of fiber-reinforced plastics articles, using a mixture of one of polyurethane, unsaturated polyester, epoxy resin an polyamide with one or more liquid components including, for example, an isocyanate component and an active hydrogen component with the addition of a reinforcing filler such as, for example, glass fibers. By way of example, when by the use of an R-RIM (Reinforced-Reaction Injection Molding) technique polyurethane is injected into a molding cavity through a gate, defined in the mold assembly, in the form as mixed with a first liquid component of polyisocyanate, a second liquid component of polyol and milled glass fibers, a molding of foamed polyurethane reinforced by the milled glass fibers can be obtained and can be used as an automobile bumper. In such case, the milled glass fibers can readily be oriented in a direction conforming to the direction in which the plasticized molding material through the gate, and therefore, although a gate-molded portion of the molded article can exhibit a reduced coefficient of linear expansion in such direction by the presence of the reinforcing fibers, a desirable shrinkage occurs in a direction perpendicular to such direction, that is, in a widthwise direction.
In the prior art injection molding apparatus, it is a general practice to employ the gate having a thickness equal to or greater than 0.5 times the thickness of a portion of the resultant molding adjacent the gate and also to employ a gate land having a length equal to about 4 times the thickness of the gate. While the cross-sectional area of the gate is generally determined in consideration of the speed (2 to 5 m per second) at which the molding material appropriate for use in injection molding is injected, the deflashing can be facilitated by the employment of the gate having a thickness so increased as it will not hamper the deflashing and of the gate land of a length as reduced as possible.
However, it has been found that, with the prior art design of the gate, shrinkage of the gate-molded portion of the resulting molding in the widthwise direction tends to extend to the molded article to such an extent as to result in surface deformation (i.e., undulated surface) of the molded article.